1. Field of the Invention
The present invention relates to an optical pickup in which an objective lens holder is slidably and pivotably supported by a support shaft.
2. Description of the Prior Art
FIGS. 1 to 4 show a conventional optical pickup of the type described above. In this conventional optical pickup, a tubular projection 1b extends downward from the central portion of a lower surface 1a of a base 1. A pair of first yokes 1d and 1e oppose each other near the center of an upper surface 1c of the base 1 through a support shaft 3 having its lower end inserted into an inner hole 2 of the projection 1b. A pair of second yokes 1f and 1g similarly oppose each other near the periphery of the upper surface 1c of the base 1 through the support shaft 3.
The base 1, the projection 1b, the first yokes 1d and 1e and the second yokes 1f and 1g are integrally formed of a magnetic material. An elongated optical window 4 is formed in the base 1 so as to allow a laser beam to pass therethrough.
A pair of first magnets 5a and 5b are mounted on the upper surface 1c of the base 1 so that they extend along the outer circumferential surfaces of the first yokes 1d and 1e at predetermined distances therefrom. A pair of third yokes 6a and 6b are mounted on the upper surfaces of the first magnets 5a and 5b, respectively. Pairs of rectangular, second magnets 7a and 7b, and 8a and 8b are fixed to the inner circumferential surfaces of the second yokes 1f and 1g, respectively.
An objective lens holder 9 consists of a flat cylindrical portion 9a and an end disk 9b mounted at the upper portion of the cylindrical portion 9a. The holder 9 consists of a non-magnetic material.
A bearing hole 10 is formed at the central axis position of the holder 9 and receives the upper portion of the support shaft 3. The objective lens holder 9 is supported to be pivotable about the axis of the support shaft 3 and to be slidable along the axial direction of the support shaft 3.
A pair of insertion holes 11a and 11b are formed in the end disk 9b of the holder 9 so as to be located at the two sides of the bearing hole 10. The distal ends of the first yokes 1d and 1e are inserted in the insertion holes 11a and 11b, respectively. A lens hole 12 is formed at a location between the insertion holes 11a and 11b and separated from the bearing hole 10. A lens barrel 14 holding an objective lens 13 therein is inserted in the lens hole 12.
A focusing coil 15 is wound circumferentially around a circumferential side wall 9c of the holder 9. Two tracking coils 16a and 16b are adhered to the surface of the focusing coil 15 so as to oppose each other through the support shaft 3 and to oppose the inner surfaces of the second magnets 7a and 7b, and 8a and 8b, respectively.
Thus, a first magnetic circuit is constituted by the first yokes 1d and 1e, the third yokes 6a and 6b, and the first magnets 5a and 5b. A second magnetic circuit is constituted by the second yokes 1f and 1g and the second magnets 7a, 7b, 8a and 8b.
When a current is supplied to the focusing coil 15, the objective lens holder 9 is slid along the axial direction of the support shaft 3 so as to perform focusing. When a current is supplied to the tracking coils 16a and 16b, the holder 9 is pivoted about the axis of the support shaft 3 so as to perform tracking.
When the tracking power for pivoting the holder 9 about the axis of the support shaft 3 is released, in order to automatically return the holder 9 to its neutral position, an elastic support member 17 as a neutral position holding mechanism is inserted between the base 1 and the objective lens holder 9.
The elastic support member 17 is obtained by welding an elastic support plate 17b consisting of silicone rubber or the like to one end of a rubber mounting metal member 17a, e.g., a metal plate.
A pair of holes 18a and 18b are formed in the base 1 so as to oppose each other through the support shaft 3 interposed therebetween and respectively receive one end of each of a pair of mounts 19a and 19b. The other end of each of the mounts 19a and 19b is received in a corresponding one of a pair of small holes 20a and 20b formed in the rubber mount metal member 17a. In this manner, the elastic support member 17 is mounted on the base 1 through the pair of mounts 19a and 19b.
The elastic support plate 17b comprises an integral assembly of first and second arms 21 and 22 which extend in two directions away from the support shaft 3 within a plane substantially perpendicular to the axis of the support shaft 3 from the side of the rubber mount metal member 17a, and a connecting portion 23 connecting the first and second arms 21 and 22. A small hole 24 formed in the central portion of the connecting portion 23 receives a small-diameter portion 25a at one end of a balancer pin 25 made of a plastic material or the like. A collar 26 is inserted into the end portion of the small-diameter portion 25a so as to couple the elastic support member 17 and the balancer pin 25.
A large-diameter portion 26b at the other end of the balancer pin 25 is fitted in a through hole 27 formed in a portion of the holder 9 which is separated from the axis of the support shaft 3, thereby coupling the objective lens holder 9 with the elastic support member 17 through the balancer pin 25.
The objective lens holder 9 and the like are housed in a housing cover 29 having an elongated opening 28 at a position corresponding to the objective lens 13.
However, in the conventional optical pickup as described above, a clearance of about 20 .mu.m is present between the support shaft 3 and the bearing hole 10 formed in the objective lens holder 9. For this reason, when the optical pickup is vibrated, the objective lens holder 9 is shaked and the objective lens 13 is abruptly displaced.
When tracking servo cannot compensate for such a displacement, a track jump is made and normal recording/reproducing cannot be performed. This means an optical pickup of the type as described above has a poor vibration resistance.